Various anti-theft caps for fuel tanks, more especially for automobile vehicles, are at present proposed on the market. Such caps comprise one or more locking members intended to engage with a flange of the mouth of the tank thus equipped. The control mechanism of these members is lodged in the deep body and operated by a key. As it is not possible infinitely to increase the diameter of such caps, the locking members must necessarily shift from the centre of the cap towards its periphery to come to engage with one or more elements of the mouth provided for this purpose. Such an arrangement presents no difficulty in so far as the mouths of the tanks to be protected are provided with an internal flange, which is the case with nearly 90% of the vehicles at present on the road. As regards the remaining 10%, which in reality represents a significant number, no satisfactory means has yet been proposed to permit the use of standard anti-theft caps. Attempts have in fact been made to use connector rings of plastic material forced around the external flange of the mouth to be equipped, but such rings have a tendency after a certain time to slacken, which eliminates the seal of the closure. It has also been proposed to use metallic rings fixed to the mouth by means of radial screws which in fact render illusory the protection offered by the lockable cap.